Stretch films are thermoplastic plastic films that are stretched and oriented during their fabrication and thermally shrink during the use. Usually, a flat film is first made by extrusion blow molding or extrusion casting and is then either stretched along both its longitudinal and transverse directions or along only one of the direction and not at all in the other directions, at a temperature above its softening point and below its melting point, where the film becomes highly elastic. Stretch films fabricated by the former approach are known as biaxially stretched films, whilst those by the latter approach are called uniaxially stretched films.
Due to their advantages such as being thin, highly transparent, tough, cold-resistant, heat-resistant, pressure-resistant, dust-resistant and waterproof, stretch films have been found wide use in the packaging of food, medicines, electronic devices and other products. However, as macromolecular chains in stretch films are highly oriented along their stretch direction(s), they suffer from low tear resistance along this/these direction(s). In particular, the commonly-used biaxially stretched films are inferior in tear resistance due to orientation of their macromolecular chains along both the stretch directions. This greatly limits the application of stretch films. Therefore, there is an imperative need for improving the tear resistance of stretch films.
Conventional methods for addressing the low tear resistance problem for stretch films can be categorized into chemical methods and physical methods. For example, Chinese
Patent Application CN201280003595.5 discloses a tear-resistant film made of a film material including an SEEPS elastomeric block copolymer having a Tm ranging from about 10° C. to 20° C. Chinese Patent Application CN200710050055.3 discloses a multi-directionally tension-resistant, tear-resistant composite film fabricated by flattening a tubular film and stretching it in one direction at a temperature that is higher than its glass transition point and lower than its viscous flow temperature to allow orientation of its macromolecules. It is then inflated to again form a tubular film, and the tubular film is spirally cut along a direction with an angle of 15-75° from the stretch direction and unfolded into a flat film in which the macromolecular orientation is inclined at an angle of 15-75° with respect to the film edges. After that, at least two such unfolded flat films are bonded together in such a manner that their macromolecular orientations cross each other, thereby forming the composite film. These methods are both subject to limitations in use and cannot be universally used for improving the production of stretch films from various existing materials and processes.